As their name implies, photomultiplier tubes are fashioned in some approximation of a tubular shape. In one common embodiment, the photomultiplier is comprised of a metal tube, the longitudinal centerline of which defines the axis of the device. In head-on type photomultiplier tubes, there is a transparent faceplate at one end of the tube that admits light or other radiation into the tube. The other end of the tube is closed with a stemplate, through which air-tight connections to various internal electrodes are made. The tube may be of circular, rectangular, or hexagonal cross-section. Rectangular or hexagonal tube cross-sections are useful when several or more photomultiplier tubes are arranged side-by-side in close proximity and a high packing density is desired.
FIG. 1 is a side-view schematic of a generic photomultiplier tube of the head-on type and of the kind wherein a metal tubular element, rather than a glass envelope, delimits the cross-sectional area of the vacuum enclosure of the device. FIG. 1 is meant simply to convey the overall structure and general features of head-on type metal-tube photomultipliers, but is non-specific about the details of the junction formed between the metal tube and glass faceplate, which is a subject of the present invention. The generic features of such a photomultiplier tube include a metal tube (102), closed at one end by a glass faceplate (104), and sealed at the opposite end by a metal stemplate (106). Electrode connections (108) are made through the stemplate. A framework or cage of electrodes (110), including various dynodes or microchannel plate(s), and anode(s) are mounted in the enclosure so formed. One electrode functions as a photocathode that upon absorption of photons emits electrons. These photoelectrons emitted from the photocathode are accelerated toward a nearby electrode by an electric field imposed between the cathode and electrode. The photocathode may be a separate electrode with a photoemissive coating, or commonly, the photocathode may be realized as a coating of photoemissive material (112) deposited on the inside surface of the faceplate. The dynodes or microchannel plates are electrically biased such that impact of an electron causes emission of several or more secondary electrons. Incident radiation (114) is transmitted through the faceplate (104) and is absorbed by the photocathode (112) to initiate the cascade of electrons that ultimately generates an anode current. The dynodes or microchannel plates provide an electron multiplication effect that is the basis of the high signal gain characteristic of photomultiplier tubes. The anode current output response to incident light depends on many factors related to the optical path of incident light and the trajectories of photoelectrons and secondary electrons. Ideally, the anode current response is independent of the position of incident light on the front face of the photomultiplier tube. However, in photomultiplier tubes constructed as shown in FIG. 1, a peripheral region (116) around the edge of the faceplate (104) that exhibits a reduced or distorted response to incident light is evident. Compared to the anode current resulting from photons incident on the center of the faceplate (104), the anode currents resulting from radiation incident upon the edge regions (116) of the photomultiplier tube front face are diminished or otherwise perturbed from the response of the central portion of the tube due to obscuration of the photocathode, non-uniformities in the optical path between the exterior side of the faceplate and the photocathode, and fringe effects in the electron multiplication cascade provided by the other electrodes. The situation is further complicated in that often the incident light is of a diffuse nature and is obliquely incident on the faceplate, resulting in multiple internal reflections within the glass faceplate or tube enclosure. This is especially true if the light is generated by a scintillator material in close proximity to the faceplate, in which case the incident radiation can be approximately isotropic, and a significant portion of the radiation will be trapped by optical confinement in the faceplate. As will be discussed with respect to the present invention, these light trapping effects can be exploited to ameliorate deficiencies in the response characteristics associated with the edge regions of the photomultiplier tube.
The spatially non-uniform anode currents associated with such edge effects create gaps or distortions in the position-dependent response characteristics of photomultiplier tubes. These edge effects, regardless of their origin or the relative contributions of various structural features and phenomena, complicate the use of such photomultiplier tubes when they are grouped together side-by-side in an imaging or detector array. The present invention seeks to address these shortcomings by utilizing a design and method of fabrication that avoids or compensates for these edge effects, particularly with regard to edge shape of the glass faceplate and the manner in which it is attached to the metal tube. The contact and sealing between the glass faceplate (104) and the metal tube (102) can be made in several ways, and is an aspect of the present invention.
In some cases, instead of a faceplate, a glass envelope of hemispherical shape, or of hexagonal or rectangular cross section with a flat top, is used. FIG. 2A shows a side view and FIG. 2B shows a perspective view of a photomultiplier tube having such a glass envelope (202) that is aligned and sealed to a metal tube (204) with the opposite end closed by a stemplate (206), similar to the photomultiplier tube of FIG. 1. A photocathode (208) can be formed as coating on the inside surface of the glass envelope. Still, in such photomultiplier designs as depicted in FIG. 2, the finite wall (210) thickness of the glass envelope is the source of an edge effect, in that radiation incident at the perimeter of the envelope is not efficiently transmitted to the photocathode. Such edge effects are inherent to some degree in practically all photomultiplier tubes, thus making their use in arrays problematic.
The reduced or distorted response of the peripheral regions of a photomultiplier tube has important consequences when a number of photomultiplier tubes are assembled in a close-packed configuration as part of an array for imaging applications. FIG. 3A shows a top plan-view of the front faces of several circular-cross section photomultiplier tubes (302) in such an array (304). The maximum packing density is evidently determined by the points of contact, e.g., (306), between the metal tubes (or the glass envelope sidewalls) of adjacent photomultipliers. In imaging or detector arrays utilizing standard photomultiplier tubes, the total photosensitive area of the array, as determined by the sum of the photosensitive areas (308) of the component photomultiplier tubes (302), is less than the nominal total area of the array itself. In particular, there are gaps (310) between adjacent photomultiplier tubes (302) upon which incident radiation will not be detected, or for which the response will be substantially reduced or distorted compared to the response for light incident on central regions of the photomultiplier tube faceplate. As an example, a schematic plot of response as a function of the position of incident radiation on the front face of the photomultiplier tube is shown in FIG. 3B for a section A–A′ of FIG. 3A. The vertical axis is the localized response. Such a plot can be understood as the result of scanning a finely focused light beam probe, such as produced by a laser, across the front face of the photomultiplier tubes of the array and for which the anode current response is recorded as a function of the position of the light beam probe along a path such as section A–A′. The dips in the response curve of FIG. 3B correspond to the reduced response associated with light incident at the periphery of photomultiplier tubes or at the intervening space between adjacent photomultiplier tubes.
Edge effects and their consequent diminished or distorted response are not limited to photomultiplier tubes of circular cross section. Photomultiplier tubes of rectangular or hexagonal cross section, although compatible with higher packing densities relative to that of circular cross section photomultiplier tubes, will still nevertheless suffer from edge effects due to the finite sidewall thicknesses of the tubes and other phenomena associated with the periphery of the faceplate and photocathode. For example, FIG. 4A shows a top plan view of an array (402) comprised of photomultiplier tubes (404) with hexagonal cross-sections. Conventional photomultiplier tubes will be characterized by a photosensitive area (406) of approximately spatially uniform response that is less than the total front face area of the photomultiplier tube. A schematic plot of localized response, analogous to that of FIG. 3B, along a section B–B′ of the array of FIG. 4A is shown in FIG. 4B. As indicated in FIG. 4B, reduction or distortion of response is typical as the light beam probe is scanned between adjacent photomultiplier tubes. This feature can be problematic for imaging applications as it represents a significant—although predictable and spatially-regular —loss of signal information.
The foregoing considerations of edge effects and their impact on photomultiplier tube imaging arrays are relevant to many types of photomultiplier tubes. Photomultiplier tubes generally have several common features including a photocathode, several dynodes or microchannel plates, and one or more anodes, all of which are enclosed in a sealed, evacuated tube. There are a wide variety photomultiplier designs specifying various electrode configurations including multiple anodes and microchannel plate(s). A review of the prior art will center on aspects of photomultiplier tubes that are germane to the present invention and which relate to the geometry and method of making a seal between the glass faceplate and metal tube. It will be understood that the present invention is applicable to the wide assortment of photomultiplier tubes that share this metal tube-glass faceplate junction, which includes most head-on type, metal tube photomultipliers, irrespective of the number, type or arrangement of the internal electrodes.
The present invention can be better understood if the details of photomultiplier tube structure and assembly are appreciated. Photomultiplier tubes constructed in the head-on type configuration consist, in part, of a glass faceplate coated with a photosensitive material which constitutes the photocathode or light-sensitive element of the device. The faceplate is sealed to one end of a metal tube that is typically rectangular in cross section. The other end of the tube is sealed with a machined metal stemplate. In practice, and often to some advantage, the photocathode coating may cover other interior surfaces of the photomultiplier tube enclosure, including the inner surface of the metal tube to which the faceplate is attached, thus extending its effective area beyond the exposed interior side of the faceplate. The sealed tube forms an enclosure containing the photocathode(s), anode(s), and dynode(s) or microchannel plate(s) of the device. The photoelectric and photomultiplier effects, upon which operation of the device is based, require the interior space of the device be maintained at a sub-atmospheric (vacuum) pressure. Therefore, the integrity of the junction between the glass faceplate and metal tube must be such that a sufficiently air-tight seal is attained and persists throughout the operating life of the device. The effectiveness of the seal between the glass faceplate and metal tube depends on the geometric details of the areas where the metal and glass make intimate contact. The seal geometry also impacts the ease of manufacture of the photomultiplier tube.
A general objective of optical detector design is a device that generates an output signal utilizing as much of the incident radiation of interest as possible. To this end, radiation which has been focused, collimated, or otherwise collected from the field of view of the detector needs to be efficiently coupled to the photosensitive component of the detector. In the case of a photomultiplier, the photosensitive element is the photocathode. Thus, any radiation incident on the photomultiplier that is not coupled to the photocathode constitutes a loss in performance of the photomultiplier tube. Invariably, some of the available light is lost due to reflection, absorption, and shading effects inherent in the geometry of the detector, and thus, the optical collection efficiency is less than perfect.
Therefore, an object of photomultiplier tube design and construction is to maximize the anode current response to incident photons, while maintaining spatial uniformity of response over as large an area as possible, and without degrading the signal-to-noise ratio. In this regard, the present invention pertains to the periphery of the front face of the photomultiplier tube, where the glass faceplate and metal tube seal is made. This edge region detracts from the response, in that light incident on this area is neither efficiently nor uniformly directed onto the photocathode. Moreover, multiplication effects for secondary electron cascades initiated by photoelectrons emitted from the peripheral regions of the photocathode may be different than multiplication effects initiated by electrons stimulated by light incident on the central area of the faceplate. Because of these edge effects, either by themselves or in combination, photomultiplier tube imaging arrays will be plagued by areas of deficient or non-uniform response, thus distorting the image.
The present invention may be regarded as a solution to a general problem encountered in the design, construction, and application of photomultiplier tubes. This problem is the result of certain geometric features of photomultiplier tubes that are consequences of the way the glass faceplate is positioned with respect to the metal tube that houses the electrodes and forms the vacuum enclosure when sealed at the front end with the faceplate and opposite end with the stem plate. Commonly practiced arrangements of the faceplate and tube tend to result in portions of the device that subtend the incident illumination but which do not efficiently couple incident radiation to the photocathode.
FIGS. 5A, 5B, 5C, and 5D show various known arrangements of forming a contact between the metal tube and glass faceplate. FIG. 5A shows the photomultiplier metal tube (502) with a metal flange (504) formed at one end and to which the perimeter of the faceplate (506) is mated and sealed to the underside of the flange (504). The faceplate may include a photocathode coating (508) as shown. Similarly, the faceplate (506) can be seated atop the flange (504) as shown in FIG. 5B. The particular embodiment of FIG. 5B may provide more structural stability under vacuum loading. The juxtaposition of faceplate (506), flange (504) and metal tube (502) in FIGS. 5A and 5B provides for an adequate seal between the metal tube and faceplate due to the relatively large metal-to-glass contact area. However, from the perspective of detector performance, this arrangement is encumbered by a considerable amount obscuration. The metal flange (504) blocks a significant portion of the radiation incident on the front surface of the photomultiplier tube, thus subtracting from the active area of the device, and therefore, the light-sensitive area of such a photomultiplier tube can be significantly less than the total or cross-sectional area of the photomultiplier tube.
FIGS. 5C and 5D show arrangements of joining the faceplate (506) and metal tube (502) that are designed for reducing losses in response associated with the edge effects inherent in the photomultiplier tube geometry described with respect to FIGS. 5A and 5B, primarily by way of eliminating the flange element (504). In FIG. 5C, the side edge of the faceplate (506) makes contact with the inside wall of the metal tube (502). Relative to the arrangement of faceplate and tube shown in FIG. 5A or 5B, the design of FIG. 5C allows more of the photocathode (508) to be exposed to incident radiation. In FIG. 5D, the perimeter region (510) of the faceplate (506) sits atop the metal tube (502). Again, relative to the arrangement of faceplate and tube shown in FIGS. 5A or 5B, the design of FIG. 5D allows more of the photocathode (508) to be exposed to incident radiation.
While the designs depicted in FIGS. 5C and 5D reduce the diminished response of the edge regions, they are not conducive to making an air-tight seal between the glass front plate and metal tube due to the relatively small contact area between these two parts.
European Patent Publication EPA 1 282 150 A1 (SHIMOI) describes a design and method of sealing the frontplate to metal tube in photomultiplier tubes that is intended to partially reduce such edge effects while providing an acceptable seal between the metal tube and glass faceplate. SHIMOI offers several variations of sealing the photomultiplier tube by embedding the edges of the metal tube into the glass faceplate. For example, FIG. 6, based on a description of SHIMOI, shows the ends of the metal tube (602) are tapered to form a knife-edge termination (604). The edges of the metal tube so formed are heated by radio-frequency (RF) heating and are then aligned with and impressed into the glass faceplate (606), which fuses at the point of contact with the metal edge due to the elevated temperature of the metal. The metal tube edge (604) can then be wedged into the softened glass faceplate (606), which then hardens upon cooling, causing a fusion bond between the glass and metal. The embedded metal tube in the glass faceplate makes a good, reliable air-tight seal. A photocathode (608) is formed on the interior side of the faceplate (606) as described in the previous examples. The degree to which the photocathode (608) is still obscured by this design depends on certain specific details of the process.
The specification and drawings of SHIMOI indicate that as a result of the fusion process, a bulge (610) forms that protrudes from the edge of the faceplate side as shown in FIG. 6. The presence of such a bulge subverts, at least somewhat, one of the objectives of the invention of SHIMOI, because it impedes intimate contact between adjacent photomultiplier tubes when they arranged in a close-packed configuration of an array. It is also evident that the response associated with the periphery (612) of the photomultiplier tube will still be diminished or distorted, in part due to the bulge that forms from the metal tube-faceplate sealing process. Thus, the embodiments of SHIMOI do not completely eliminate edge effects on photomultiplier tube response, nor do they allow near-maximum close packing densities in photomultiplier tube arrays.